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The last echo of a super-massive black hole activity

Using a brand-new radio telescope, astronomers have taken one of the best images ever made of giant bubbles produced by a super-massive black hole in a frequency range normally used by airplane pilots. The picture shows what looks like a giant balloon filled with radio emitting plasma, which exceeds the size of an entire galaxy. This balloon was slowly inflated by one of the most massive black holes in our cosmic neighborhood.

Surprisingly, black holes do not only swallow matter, but they are also able to eject a fraction of the doomed particles. The expelled matter then appears in the form of a hot plasma stream, leaving the black holes’ host galaxy at a velocity close to the speed of light. As the plasma slows down it creates a large and extremely tenuous bubble engulfing the entire galaxy and its surroundings. This plasma bubble is invisible in optical telescopes, but is very prominent at low radio frequencies.

"The result is of great importance", says Francesco de Gasperin, first author of the study that will be published in the journal Astronomy & Astrophysics, "because it shows the great potential of LOFAR and also provides the compelling evidence of the strong interaction between the galaxy’s super-massive black hole and the galaxy’s surroundings. Like symbiotic species", says de Gasperin, "the galaxy and its super-massive black hole have intimately related lives, the galaxy providing matter to feed the black hole, and the black hole returning energy to the galaxy".

The image was made during the test-phase of the new International LOFAR Telescope (ILT) at radio frequencies above 120 MHz.

“This is the first time that such high-quality images have been possible at such low frequencies", says, Prof. Heino Falcke, chairman of the board of the ILT and co-author of the study. "This is one of the most difficult regions in the sky for a radio telescope - we would not have expected to get such high-quality results so early in the commissioning phase of LOFAR."

The scientists observed a huge elliptical galaxy, Messier 87 (M87), at the centre of a galaxy cluster in the constellation of Virgo. This galaxy is 2000 times more massive then our Milky Way and hosts in its centre one of the most massive black hole discovered so far, with a mass 6 billion times that of our Sun. Far from being quiet, every few minutes this black hole swallows an amount of matter similar to that of the whole Earth, converting part of it into radiation and a larger part into powerful jets of ultra-fast particles, which are responsible for the observed radio emission.

The information that is contained in the spectrum of the radio waves provides a track record of the activity of supermassive black holes. The team found that these balloons are surprisingly young, just about 40 million years, which is a mere instant on cosmic time scales. "What is particularly fascinating", says Andrea Merloni from the Max-Planck Institute of Extraterrestrial Physics in Garching, who supervised de Gasprin's doctoral work, "is that, by measuring the power of the large scale outflow observed by LOFAR, we learn a great deal also about the violent processes of matter-to-energy conversion taking place very close to a black hole. In this particular case, the black hole seems to be much more efficient in accelerating the jet than producing visible radiation."

For the age analysis the authors also used radio data from the Very Large Array in New Mexico and the Effelsberg 100m radio telescope near Bonn, Germany.

LOFAR is a revolutionary instrument able to detect radio waves with wavelenghts up to 30-meter. This long radio waves are normally generated by many human activities as broadcasting radio, radar signals or satellite communications, but they are also emitted in deep-space by exotic objects such as accreting black holes, rotating neutron stars and supernovae. To detect these waves, LOFAR uses thousands of antennas spread all over Europe and combines the signals in a supercomputer located in the Netherlands. The 100 Gigabit per second of data flowing from all antennas are analyzed simultaneously and in real-time to provide the most detailed images ever done at these frequencies.

International LOFAR Telescope operations are coordinated by ASTRON, the Netherlands Institute for Radio Astronomy, on behalf of a consortium consisting of the Netherlands, Germany, France, the UK, and Sweden. Many of the technological solutions developed for LOFAR, in particular the calibration of phased-arrays as well as large-scale data transport and processing, will be highly relevant for future radio telescope projects such as the Square Kilometer Array (SKA).

Francesco de Gasperin performed the study as part of his PhD work at the Max Planck Institute for Astrophysics and at the Excellence Cluster Universe. De Gasperin is now a postdoctoral researcher at the University of Hamburg.